This invention is related in general to digital-to-analog (D/A) converters, and more particularly to D/A converters using a resistor strip and an auto zeroing function in the conversion between digital and analog signals.
Digital-to-analog converters (DAC) find a wide variety of applications in converting digital signals to corresponding analog voltages. Digital-to-analog converters also find a more specific application in converting a succession of processor-generated digital words to analog voltages for generating tones, sounds, voice signals, etc.
The prior art is replete with various techniques for converting digital signals to corresponding analog voltages. Numerous different types of conversion circuits are well known to those skilled in the art. One technique for converting a digital signal to a corresponding analog voltage is by way of a resistor strip and a switch arrangement. The resistor strip includes a number of series-connected resistors for providing different analog voltages. The switches are utilized for selectively extracting a voltage from one of the nodes of the resistor strip. The particular switch or switches that are operated is a function of the logic states of the bits of the digital word being converted. With this technique, it is common to utilize a successive approximation register (SAR) circuit to make successive attempts at selecting the correct analog voltage from one of the resistor strip nodes, such that the voltage uniquely matches the value of the digital word.
The number of resistors required to complete the conversion process is related to the number of bits comprising the digital word. A twelve-bit digital word represents 4,096 different combinations of logic states, and in a conversion process can correspond to 4,096 different analog voltage levels. In the simplest case, a resistor strip having 4,095 equal-value resistors connected in series could be used to produce the same number of analog voltage levels, including ground. Various other techniques and schemes are readily known to those skilled in the art for reducing the number of resistors required in order to produce the requisite number of analog levels as a function of the number of bits in the digital word.
The fabrication of resistor strips in a semiconductor chip for a DAC is well known to those skilled in the art. FIG. 1 illustrates a portion of a semiconductor resistor strip 10 fabricated with a polycrystalline silicon (polysilicon), appropriately doped to provide the desired number of ohms per square. The polysilicon resistor strip 10 is formed by depositing a layer of polysilicon 12, masking the same and etching the masked polysilicon in the general shape shown in FIG. 1. A number of nodes or taps, one shown as reference numeral 14, are formed in the semiconductor material. In other variations, taps or nodes may be formed on both sides of the resistor strip 10. Each tap, such as voltage tap 14 and voltage tap 16, are spaced apart identically, as is the physical distance between tap 16 and 18, as well as the distance between tap 18 and tap 20 and so on. In this manner, the voltage drop between tap 14 and tap 16 is preferably identical to the voltage drop between tap 16 and tap 18, as well as that between tap 18 and tap 20.
At the bottom of the resistor strip 10 there is a contact 26 connected by interlevel metal to a ground conductor 28 which is shown in phantom. The ground conductor 28 can constitute ground itself, or a conductor that is connected to a common circuit ground, denoted in FIG. 1 as reference numeral 30.
As can be appreciated, in a unipolar conversion process, when the digital bits of a word all have a zero value, the corresponding analog voltage is 0.00 volts. In this situation, the bottom-most voltage tap 26, which is connected to ground, is generally selected by switches (not shown) for coupling a zero voltage to the output of the DAC. A digital value of 00 . . . 001 will cause the voltage at tap 20 to be selected by the switch arrangement. Tap 20 represents the smallest non-zero analog voltage generated by the resistor strip 10, since it is located next to the ground tap 26. The voltages between the resistor taps may be in. the low millivolt range, such as one or two millivolts. As such, not only is the uniformity of the spacing between the taps necessary, but also a ground tap 26 that is in fact 0.00 volts.
In practice, the interlevel contact structure inherently includes some resistance, as does the ground conductor 28. As such, the voltage at the bottom tap 26 is not ideally zero volts, but rather may be some tens or hundreds of microvolts. Accordingly, when carrying out the digital-to-analog conversion, there is an inherent error because the ground tap 26 is not ideally zero volts. This error is also present in the conversion of each of the other combination of bits, as the voltage at each tap 14-20 is generated with respect to ground in the DAC of FIG. 1. When high precision is required in the conversion process, the error generated because of the non-zero voltage at tap 26 causes the accuracy and linearity to be compromised.
From the foregoing, it can be seen that a need exists for a technique in reducing the non-linearities and inaccuracies caused by DAC resistor strips which utilize a ground tap at one end thereof. Another need exists for a DAC utilizing a grounded resistor strip, but where the conversion process is independent of any undesirable non-zero voltage at the ground tap.
In accordance with the principles and concepts of the invention, there is disclosed a D/A circuit that overcomes the disadvantages and shortcomings of the prior art techniques. In accordance with a disclosed embodiment of the invention, a resistor strip is fabricated in a semiconductor material with a plurality of voltage taps. A current that passes through the resistor strip generates a voltage at each tap. The difference in voltage (xcex94v) between each tap is ideally equal, thereby providing equal increments of voltage along the resistor strip taps. One end of the resistor strip is preferably grounded or coupled to a common or reference voltage. However, rather than utilizing the reference of zero volts, namely, the ground voltage, the reference voltage utilized is at the first non-zero voltage tap formed in the resistor strip. Accordingly, in order to provide, for example, 256 different analog voltages, 257 voltage taps are provided, with the bottom non-grounded voltage tap of the resistor strip representing the reference. This non-zero reference voltage at the first resistor strip tap can be selected when the logic states of the digital bits are all zeroes.
In accordance with an additional feature of the invention, the analog voltage output from the resistor strip, via a switch arrangement, is capacitor-coupled to an amplifier. The amplifier is configured to provide automatic zeroing with respect to the non-zero voltage of the first or reference resistor strip tap. With this arrangement, any imperfect grounding or ground resistance terminating the resistor strip to ground is overcome and otherwise eliminated from affecting the conversion process. Linearity of the conversion process is thus independent of the quality of the ground connection to the resistor strip.
The semiconductor resistor strip of the invention is constructed with multiple taps and connected to high impedance conversion circuits so that current entering the resistor strip from a supply voltage does not exit the resistor strip by way of the voltage taps. Hence, the contact resistance that may be present in the voltage tap contacts does not affect the analog voltage selected from the voltage taps. According to one aspect, the voltage taps of the resistor strip are formed as arms on the sides of the polysilicon strip, so that the current flowing through the bulk of the resistor strip is unaffected by any contact resistance inherent in the voltage taps.
Once the polysilicon strip is formed with the voltage tap arms extending from the sides thereof, a mask is formed over the polysilicon body portion of the resistor strip, except for the arms. The chip is then subjected to a siliciding process. With this technique, only the voltage tap arms are silicided for allowing a quality contact to be made to the resistor strip. In the masking of the resistor strip, the ends thereof which employ current carrying-contacts may also be exposed so as to be silicided to thereby provide low-resistance current-carrying contacts.